Electrolytic process



nited States ?atent ELECTROLYTIC PROCESS Eugene Wainer, Cleveland Heights, Ohio, assignor by mesne assignments, to Horizons Titanium Corporation, a corporation of New Jersey Filed May 10, 1957, Ser. No. 658,237

3 Claims. (Cl. 204- 64) No Drawing.

processes directed to the recovery of these metals in substantially pure form has heightened considerably.

One technique for producing the designated metals which has been extensively investigated is the electrolytic recovery of the desired metal as a cathode deposit in an electrolytic process employing a fused salt electrolyte, for example, as described in United States Patent 1,835,025. Another technique often attempted for producing zirconium as relatively pure metal involves the reduction of compounds of these metals by pyrometallurgical methods, for example, by the Kroll process described in United States Patent 2,205,854, which has been adapted to other metals such as hafnium and thorium.

Both of these avenues generally require a relatively pure halide raw material, such as ZrCl, or K ZrF or other sources of zirconium, or the corresponding hafnium or thorium source material; and the preparation of such source materials represents an economic disadvantage in the practice of both the known electrolytic and the known pyrometallurgical processes for recovering v the pure metals.

I have now discovered a process which combines many of the advantageous features of the prior art processes in a novel and economically attractive procedure in which relatively inexpensive, available raw materials are first converted to a crude impure metallic product in a pyrometallurgical step, and the resulting intermediate product is then processed to yield the desired pure metal by an electrolytic refining process.

In the description which follows, specific reference will be made to zirconium, by way of illustration, but it should be understood that whatever is said for zirconium is equally applicable to hafnium and to thorium, and that the description is to be taken to apply equally to zirconium, hafnium and thorium.

In the first portion of my process, in which an impure metallic product is obtained pyrometallurgically, the

'source of the desired metal is preferably an ore, concentrate, or oxide of the metalibut it is also possible to use any available compound of the named metals which, on heating to a sufncient temperature in contact with a carbonaceous material under a subatmospheric pressure, yields an impure metallic product containing upwards of 80% of the desired metal, associated with substantial amounts of carbon and lesser amounts of other contaminants.

The impure metallic product, which is to serve as the 2,937,979 Patented May 24, 1960 'ice consumable anode, is preferably prepared by reacting an oxide of zirconium or hafnium with either carbon or a carbide of the same metal, in much the same manner as described in United States Patent 2,205,386. However, I have found that as reported by the patentee in volume of the Transactions of the Electrochemical Society, the complete removal of the last traces of both carbon and oxygen, when the process is carried out in accordance with the teachings of the patent, presents a problem of major proportions. Instead of endeavoring to remove the last traces of either of these contaminants, and pro duce the pure metal directly, I find that the reaction between the carbonaceous material and the oxygenous material may be carried out to a limited extent, sufficient to produce as the product a metal contaminated by as much as 3 to 5% carbon and as much as 3 to 5% oxygen. In general, such products may be obtained by effecting the reaction at lower temperature, lower vacuum, or for shorter intervals, than are described in the patent.

One means of production of such compounds is as follows: 125 parts of commercially pure zirconium oxide in 325 mesh particle size, 25 parts of 325 mesh calcined petroleum coke, 0.5 part of methyl cellulose and 15 parts of water are thoroughly mixed and kneaded together until a plastic mass is obtained. The mix is then pressed into pellets roughly /2" in diameter and /2" thick at 4,000 pounds per square inch. After thorough drying at 110 C., the pellets are heated in an actively pumping vacuum in which the pressure is maintained at microns or less to a temperature of 1700 to 1800" C. and maintained in such ambient for one hour. The reaction is cooled under vacuum and the product is found to weigh 103 grams, and analyzes: 4.2% carbon, 4.8% oxygen, 88% zirconium, 3% others.

Advantageously, the procedure described in the patent may be further modified by the addition of silicon or silicon carbide to the mixture of reactants to assist in the removal of oxygen from the reaction mixture without a corresponding change in the carbon content of the resulting product. The addition may be made either to the initial mixture of reactants, in which case the silicon added will be converted to silicon monoxide which escapes from the mixture as a volatile product; or the addition of silicon may be made to the impure metal product of the carbonaceous-'oxygenous reaction mixture, after crushing that product; and the resulting mixture may then be heated to remove further amounts of oxygen and possibly carbon.

The crude metallic product resulting from the above described treatment has been found to be amenable to refining by means of a fused salt electrolysis in which it is employed as a consumable anode material. The electrolysis may be carried out in any suitable apparatus known in this art, for example in the apparatus shown in any of United States Patents 2,748,073, 2,734,855 or 2,783,195. The electrolytic cell is preferably a vessel provided with means for maintaining the contents of the vessel in molten condition, and with means for maintaining any desired atmosphere over the melt.

The melt constituting the electrolyte in the electrorefining portion of my process consists essentially of at least one alkali metal halide other than the fluoride and preferably in admixture with between about 5% and 35% of a double fluoride of an alkali metal and the metal to be electrorefined. Suitable melts have been formed of mixtures of NaCl or RG1 or both NaCl and KCl with K ZrF for example, with amounts between 5 and 35 by weight of the latter. 7

The electrolyte employed is maintained molten at temperatures below which any fuming or electrolyte decom position occurs. Temperatures between 700 C. and 950 C. have been found to be generally suitable. The

electrolysis is conducted at cathode current densities of between 50 and 500 amperes per square decimeter, and at voltages between 1.0 and 11.0 volts. When the process is operated properly, no gaseous products are evolved from the electrolysis. V 3

The metallic zirconium, hafnium or thorium, is recovered as a pure metal deposited on a cathode of stainless steel, molybdenum, Hastelloy C or other suitable metal from which it is readily separated by physical means. The deposited metal and the salt adhering to it are me chanically removed, crushed and the crystals of purified metal recovered by aqueous means.

The following example will serve to further illustrate the present invention by describing a. preferred specific example of carrying out the present invention.

As described above, a suitable reaction mixture of finely divided (minus 200 mesh, Tyler standard) zirconia and carbon may be prepared by blending the-two ingredients in proportions such that slightly more than one mol equivalent of carbon is present for each mol equivalent of oxygen. The mixture may then be charged into a container, suchas a refractory boat or a graphite crucible, and heated under reduced pressure in a vacuum furnace. The carbon monoxide formed bythe reaction between the charge materials is removed by means of a vacuum pump, or in a stream of an inert gas such as argon or helium. After the reaction has proceeded to the extent desired, the application of heat is discontinued and the reaction product is permitted to cool.

When cool, the reaction product, either in the form of a single piece or as several pieces, or the products of several such reactions are charged into an electrolytic cell in which they constitute the anode. An electrolysis is performed in which the electrolyte is a fused mixture of at least one alkali metal halide and a double fluoride of an alkali metal and zirconium. The electrolysis is conducted in the manner described by me in a paper presented in a symposium on zirconium and published by the American Society for Metals under that title in 1954. Preferably, the electrolyte is one composed initially of 65 weight percent of NaCl and 35 Weight percent of K ZrF The electrolyte may be purified as described in United States Patent 2,782,156 to improve the quality of the metal product.

Other electrolytes containing zirconium ions, provided usually by either simple zirconium halides or complex zirconium halides, or mixtures of such halides in suflicient amounts for the bath to contain between 5% and by weight of zirconium ion, have also been used in the practice of my process. 7

The electrolysis has been carried out with the bath at temperatures between about 700 C. and 950 C., and at initial cathode current densities of between 200 and 300 amperes per square decimeter, and under an inert atmosphere. As a result, an extremely pure zirconium metal has been deposited at the cathode.

As indicated above, silicon in the form of elemental silicon or as silicon carbide may be added to the mixture either before or after some reaction has been eflected between the carbonaceous and the oxygenous material. By this means I have found it possible to control the oxygen and carbon contents independently of one another.

Since the removal of carbon and oxygen in the subsequent electrorefining operation may require somewhat diflerent electrical operating conditions, it has been found to be advantageous to remove the oxygen independently of the carbon in the Pym-metallurgical portion of the process and to thereby simplify the electrorefining process.

I claim:

1. The process for producing a metal of the group consisting of zirconium, hafnium and thorium which comprises: preparing an impure metallic product rich in the desired metal by providing a mixture proportioned to provide slightly more than one mol equivalent of carbon for each mol equivalent of oxygen and consisting of (1) an oxygen compound of the metal, from the group of compounds consisting of the oxides and compounds which yield the oxides on heating, with (2) a carbonaceous material from the group consisting of elemental carbon and a carbide of the metal to be produced; and maintaining the mixture under conditions of pressure and temperature for a time suificient that the carbon monoxide evolved from the mixture producesa contaminated metallic residue containing between 3% and 5% by weight of oxygen, between 3% and 5% by weight of carbon, and more than85% by weight'of the metal; and electrorefining the contaminated metallic residue as the anode'in a fused salt electrolyte consisting essentially of at least one alkali metal halide other thah the fluoride and a double fluoride of an alkali metal and the metal to be recovered as a refined product, by passing an electrolyzing current through the electrolyte and between said anode material and a cathode, whereby the desired metal is recovered as a pure metal deposited on the cathode.

2. The process for producing a metal of the group consisting of zirconium, hafnium and thorium which comprises: preparing an impure metallic product rich in the desired metal by providing a mixture proportioned to provide slightly more than one mol equivalent of carbon for each mol equivalent of oxygen and consisting of (1) an oxygen compound of the metal, from the group of compounds consisting of the oxides and compounds which yield the oxide on heating, with (2) a carbonaceous material from the group consisting of'elemental carbon and a carbide of the metal to be produced, and (3) a material from the group consisting of silicon and silicon carbide; and maintaining the mixture under conditions of pressure and temperature for a time sutficient that the carbon monoxide evolved from the mixture produces a contaminated metallic residue containing between 3% and 5% by weight of oxygen, between 3% and 5% by weight of carbon, and more than by weight of the metal; and electrorefining the contaminated metallic residue as the anode in a fused salt electrolyte consisting essentially of at least one alkali metal halide other than the fluoride and a double fluoride of an alkali metal and the metal to be recovered as a refined product, by passing an electrolyzing current through the electrolyte and between said anode material and a cathode, whereby the desired metal is recovered as a pure metal deposited on the cathode.

3. "The process for producingpure zirconium which comprises: preparing an impure metallic product rich in; zirconium by providing a mixture proportioned to provide; slightly more than one mol equivalent of carbon for each mol equivalent of oxygen and consisting of zirconia, and. a carbonaceous material from the group consistingof.v elemental carbon and a carbide of zirconium; and main-' taining the mixture under conditions of pressure and tern-1 perature for a time sufficient that the carbon monoxide evolved from the mixture produces a contaminated metallic residue containing between 3% and 5% of oxygen, between 3% and 5% of carbon, and more than 85% of the zirconium; and electrorefining the contaminatedmetallic residue as the anode in a fused salt electrolyte consisting essentially of sodium chloride and potassium fluozirconate (K ZrF by passing an electrolyzing current between a solid cathode and the impure anode, each of which is in electrical contact with said electrolyte, and recovering'the'pure zirconium metal cathode deposit.

' References Cited in the file of this patent UNITED STATES PATENTS Steinberg et al. May 14, 1957; 

1. THE PROCESS FOR PRODUCING A METAL OF THE GROUP CONSISTING OF ZIRCONIUM, HAFNIUM AND THORIUM WHICH COMPRISES: PREPARING AN IMPURE METALLIC PRODUCT RICH IN THE DESIRED METAL BY PROVIDING A MIXTURE PROPORTIONED TO PROVIDE SLIGHTLY MORE THAN ONE MOL EQUIVALENT OF CARBON FOR EACH MOL EQUIVALENT OF OXYGEN AND CONSISTING OF (1) AN OXYGEN COMPOUND OF THE METAL, FROM THE GROUP OF COMPOUNDS CONSISTING OF THE OXIDES AND COMPOUNDS WHICH YIELD THE OXIDES ON HEATING, WITH (2) A CARBONACEOUS MATERIAL FROM THE GROUP CONSISTING OF ELEMENTAL CARBON AND A CARBIDE OF THE METAL TO BE PRODUCED, AND MAINTAINING THE MIXTURE UNDER CONDITIONS OF PRESSURE AND TEMPERATURE FOR A TIME SUFFICIENT THAT THE CARBON MONOXIDE EVOLVED FROM THE MIXTURE PRODUCES A CONTAMINATED METALLIC RESIDUE CONTAINING BETWEEN 3% AND 5% BY WEIGHT OF OXYGEN, BETWEEN 3% AND 5% BY WEIGHT OF CARBON, AND MORE THAN 85% BY WEIGHT OF THE METAL, AND ELECTROFINING THE CONTAMINATED METALLIC RESIDUE AS THE ANODE IN A FUSED SALT ELECTROLYTE CONSISTING ESSENTIALLY OF AT LEAST ONE ALKALI METAL HALIDE OTHER THAN THE FLUORIDE AND A DOUBLE FLUORIDE OF AN ALKALI METAL AND THE METAL TO BE RECOVERED AS A REFINED PRODUCT, BY PASSING AN ELECTROLYZING CURRENT THROUGH THE ELECTROLYTE AND BETWEEN SAID ANODE MATERIAL AND A CATHODE, WHEREBY THE DESIRED METAL IS RECOVERED AS A PURE METAL DEPOSITED ON THE CATHODE. 